1. Field of the Invention
The present invention relates to a discharge lamp for a vehicle. More particularly, the present invention relates to a discharge lamp for preventing a generation of dazzling light when a lead wire connected to an electrode is arranged in an outer tube. The present invention also relates to a downsizing of a discharge lamp for a vehicle.
2. Background
In a vehicle headlight, for example, an incandescent lamp (incandescent bulb) or a halogen lamp (halogen bulb) is used for a light source. Alternatively, a discharge lamp (discharge bulb) is used for the light source.
In a headlight in which an incandescent lamp or a halogen lamp is used as the light source, a filament of the incandescent lamp or the halogen lamp is substantially uniformly luminous and formed into a rod-shaped luminous portion. Accordingly, when the incandescent lamp or the halogen lamp is used for a reflection type lighting device in which a reflector is used, it is easy to execute a light distribution control by a shape of a reflecting face of the reflector. Using the incandescent lamp or the halogen lamp as the light source has the above advantages.
On the other hand, when a vehicle headlight, in which a discharge lamp is used as the light source, the following advantages are provided. Since a quantity of light of the discharge lamp is larger than that of the incandescent lamp or the halogen lamp, it is possible to enhance a luminance. In addition, a life of the discharge lamp is longer than that of the incandescent lamp or the halogen lamp.
As described above, the luminance of the discharge lamp is higher than that of the incandescent lamp or the halogen lamp and the life of the discharge lamp is longer than that of the incandescent lamp or the halogen lamp, the vehicle headlight having the discharge lamp has recently come into wide use.
There is a discharge lamp including a luminous tube made of transparent quartz glass in which a pair of electrodes are held and rare gas is filled. However, the luminous tube made of quartz glass is corroded by metallic halide filled in the luminous tube. Accordingly, the blackening and the devitrification are caused in the last stage of its life and it becomes impossible to obtain a proper light distribution and the luminance is lowered.
Therefore, in JP-A-2004-103461, it is proposed to use a ceramic luminous tube made of a transmission type ceramics having a high heat resistance property not corroded by metallic halide, in the discharge lamp, instead of the above glass luminous tube.
In the discharge lamp described in JP-A-2004-103461, a pair of electrodes are held by the ceramic luminous tube and respectively connected to the first and the second lead wire. A portion of the first lead wire and a portion of the second lead wire are respectively joined to and sealed by both end portions of the ceramic luminous tube. Therefore, an airtightly closed space is formed in the ceramic luminous tube. The airtightly closed space formed in the ceramic luminous tube is filled with gas such as rare gas and metallic halide. The ceramic luminous tube is covered with an outer tube made of glass.
The first lead wire positioned on the front side is protruded to the front from the outer tube. The third lead wire is connected to a front end portion of the first lead wire. The third lead wire is formed out of a horizontal portion extending in the horizontal direction and a vertical portion extending upward from a front end portion of the horizontal portion. The vertical portion is connected to a front end portion of the first lead wire. The horizontal portion of the third lead wire is attached with an insulating sleeve.
A rear end portion of the horizontal portion the third lead wire is connected to a socket. A end portion of the second lead wire located on the rear side is also connected to the socket.
In the discharge lamp composed as described above, the socket is attached to the reflector and arranged in a lighting device space formed by a lamp housing and a cover. In the discharge lamp, when voltage is impressed upon a pair of electrodes, a discharge is executed in the luminous portion and light emits from the luminous portion. The emitting light is reflected on the reflection face of the reflector and irradiated to the front of a vehicle.
However, in the conventional discharge lamp described in JP-A-2004-103461, since the third lead wire is arranged outside the outer tube, the light emitting from the luminous portion to the third lead wire side and the light emitting from the luminous portion and reflected on the reflection face of the reflector reach a horizontal portion (insulating sleeve) of the third lead wire and become dazzling light (glare light) being reflected by the third lead wire in a direction not intended.
In addition, because the third lead wire is arranged outside the outer tube, it is necessary to provide a connecting portion of the socket with the horizontal portion of the third lead wire at a corresponding position outside the outer tube. Accordingly, dimensions of the socket are increased. When the dimensions of the socket are increased, under the condition that the discharge lamp is attached to the reflector, an area occupied by the socket with respect to the shape of the reflector is increased. Accordingly, the reflecting face of the reflector is reduced corresponding to the increase in the area occupied by the socket and it becomes impossible to effectively utilize the light emitting from the discharge lamp.